The present invention relates generally to multi-component spraying systems and, more particularly, to an internal mix, air-assisted, airless atomization, plural component spraying system and method.
Multi-component spraying systems are used in manufacturing plastic articles by applying resinous materials to a mold or preform for an article. In such systems, a liquid resin and a catalyst for the resin are formed into spray particles directed to a substrate where the catalyst and resin react and harden to form the article. In such applications, the resin and catalyst components are preferably mixed together; and the mixture is sprayed onto the substrate. For example, in manufacturing articles with polyester resin, a catalyzing agent for the polyester resin is mixed with the resin; and the resin-catalyst mixture is applied to the substrate. In internal mix systems, the resin and catalyst are mixed within the spraying apparatus; and the mixture is atomized by a spray nozzle and directed onto the substrate. In external mix systems, the resin and catalyst are mixed externally of the apparatus after the resin and catalyst have been atomized. In both external mix and internal mix systems, complete and thorough mixing of the resin and catalyst is important to avoid non-uniform hardening of the resin on the substrate and other undesirable results.
To effect mixing, internal mix systems have incorporated fluid division mixers to intermingle the plural components of a catalyzed resin system within the spray gun. An additional mixing stage in the form of a baffle is also frequently incorporated into such internal mix systems to provide for this component intermingling within the spray gun. Such an internal mix system is shown in U.S. Pat. No. 3,759,450, and can be effective where further mixing of the resin and catalyst can be effected by spraying. Internal mix systems have also incorporated swirl chambers to effect component mixing within the spray gun. In internal mix systems using swirl chamber mixers, the resin and catalyst are injected into the swirl chamber at high velocities to create turbulence and intermixing of the two components.
Such internal mix systems are not entirely satisfactory, however, in plural component spraying systems. Because the chambers in which mixing occurs are open to the fluid pressures of both the resin delivery system and the catalyst delivery system, the fluid pressures produced by both systems must be approximately equal to avoid either of the fluid components from being forced through the orifice by which the other component is introduced into the mixing chamber. This problem is particularly acute in internal mix systems with swirl chambers where the orifices by which the resin and catalyst are introduced into the swirl chamber are frequently opposed and direct their high velocity streams at each other to effect greater turbulence and mixing. Where the orifices by which the resin and catalyst are injected to a swirl chamber are not so opposed, mixing of the resin and catalyst is not so well effected because of the velocity of the components and their short exposure to mixing. If the pressures by which the resin and catalyst are introduced into internal mix systems are not carefully controlled to be about equal and the resin and catalyst delivery systems are not simultaneously operated, either resin or catalyst may be forced into the fluid delivery system components of the other component where, because of the mixing of the components, catalyzed resin may harden and require replacement of system components.
Because internal mixing takes place with a hand-held and manipulated spray gun in commercial plural component spraying systems of the internal mix type, mixing of the two components must be effected within a small, lightweight, and easily manipulated structure. The limitations of size and weight inhibit thorough mixing by the internal mix structure within the spray gun, and further mixing must be effected in the operation of the spray gun for effective manufacture of articles from internal mix, plural component spraying systems.
In many spraying systems, large quantities of pressurized air are used to atomize the liquid components. Such systems are expensive to operate and have a number of operational inadequacies. It is expensive to compress air, and the large quantities of compressed air used by existing systems impose a significant operating cost on the system. In addition, the blast of compressed air used to atomize the liquid components carries a significant quantity of spray particles away from the substrate, wastes the expensive resin and catalyst, creates an unclean spray area and sometimes requires overspray collection systems, and contributes to the problem of operating such manufacturing operations safely. Furthermore, the use of large quantities of air during operation of the system can often create an undesirable spread of fumes.
In order to overcome some of the inadequacies attending the use of pressurized air to atomize components dispensed from a spraying apparatus, spraying systems have been developed which incorporate airless atomization techniques.
In prior airless atomization devices, an airless spray nozzle has been used to atomize liquid materials which are pumped at high pressure, that is, pressures generally exceeding 500-600 p.s.i. and more frequently in excess of 800 p.s.i., typical operating pressure being 1000-1500 p.s.i. The most commonly used airless nozzle includes an internal, hemispherical passage termination which is cut through by an external V-shaped groove to form an elongated, elliptical-like orifice. Liquid material pumped at high pressures through such a spray nozzle is forced by the hemispherical termination of the passageway to converge in its flow at and through the elongated orifice. Because of the converging flow at the orifice, the liquid material is expelled through the orifice into an expanding fan-like film which breaks into spray particles which are carried by their momentum to the article target. Flow convergence at the elongated orifice is believed to be basic to the fan-like film produced in airless atomization. The mechanism by which the fan-like film is broken into spray particles is apparently a combination of its velocity and interaction with the relatively quiescent air through which the film is projected and, generally, other hydrodynamic factors.
With viscous fluids, such as the resins used in plural component spraying systems to manufacture plastic articles, high pressures of 1000 to 1500 p.s.i. are required. Such high operating pressures impose a strain on system components and a reduction in their reliability, require generally expensive components in the fluid delivery systems, and contribute to the problem of operating such systems safely. In addition, the use of high operating pressure frequently results in porosity in the manufactured product because of air entrapped by the viscous materials during their deposition. In internal mix spraying systems using airless atomization, such very high pressures are also required in the catalyst delivery system to avoid mixing of the catalyst and resin within components of the spraying apparatus other than the mixing chamber.
Even at high pressures, however, such fan-like films, because they are formed by the convergence of the fluid, include heavy streams at their edges which are referred to in the trade as "tails". Because of the heavy stream-like flow in the "tails", spray particles formed from these edge portions of the expanding fan produce particles which are generally unacceptably large. Past efforts to solve the problem of the "tails" attending the use of airless spray nozzles have included the insertion of a "preorifice" immediately behind the elongated, elliptical-shaped orifice to concentrate a greater portion of the flow in the central portion of the fan. Although preorifices are helpful, they are not fully satisfactory, adding another source of clogging to the spray gun and another variable factor to be integrated into system operation. Because of the problems attendant the use of airless atomization, uniform mixing of catalyst and resin is not always effected within the spray of particles formed by the airless spray nozzle; and thorough mixing of catalyst and resin frequently depends upon the manipulation of the spray gun by the system operator.
Another effort to avoid the problems attending the use of airless spray nozzles and to provide greater flexibility in their use has been to assist the airless atomization with the use of compressed air jets. The air-assisted, airless atomization sprayers generally in use in prior plural component spraying systems have included a plurality of very small, precisely located, compressed air orifices positioned around the airless spray nozzle to direct a plurality of air jets at the fan-shaped film formed by the airless nozzle and, particularly, at the "tails" formed at the extremities of the film. The plurality of jets of compressed air forms areas of low-pressure and turbulence through which the extremities of the fan-shaped film were projected, thus atomizing the "tails" into spray particles of acceptable size. Although the use of such small, precisely located air orifices can improve the mixing of the resin and catalyst in an internal mix, plural component system, their effects are generally localized to obtain atomization of the "tails".